1. Field of the Invention
The invention relates generally to exhaust emission control system and method for an internal combustion engine, and, more particularly, to such exhaust emission control system and method that purify exhaust emissions by using a NOx storage/reduction catalyst, and have a function or step of determining the degree of degradation of the NOx storage/reduction catalyst. The NOx storage/reduction catalyst is adapted to selectively store NOx contained in exhaust gas, through adsorption, absorption or both of them, when the air/fuel ratio of the exhaust gas flowing into the catalyst is lean, and reduce and remove the stored NOx by using reducing components in the exhaust gas when the air/fuel ratio of the exhaust gas flowing into the catalyst becomes equal to the stoichiometric air/fuel ratio or a rich air/fuel ratio.
2. Description of Related Art
An exhaust emission control system is known which purifies exhaust gas of NOx contained therein, by using a NOx storage/reduction catalyst that selectively stores NOx in the exhaust gas, through adsorption, absorption or both of them, when the air/fuel ratio of the exhaust gas flowing into the catalyst is lean, and reduces and removes the stored NOx by using reducing components contained in the exhaust gas when the air/fuel ratio of the exhaust gas flowing into the catalyst becomes stoichiometric or rich.
However, the NOx storage/reduction catalyst undergoes degradation or deterioration for various reasons, and its capability of removing NOx from exhaust gas is reduced due to the degradation. For example, in the case where a sulfur component is contained in a fuel of the internal combustion engine, the NOx storage/reduction catalyst stores sulfur oxides in the exhaust gas, which are generated by combustion of the sulfur component in the fuel, in substantially the same manner in which the catalyst stores NOx. If the amount of the sulfur oxides (SOx) stored in the NOx storage/reduction catalyst is increased, the NOx storage capacity of the NOx storage/reduction catalyst is reduced by a degree corresponding to the stored SOx amount, and the amount of NOx that passes through the NOx storage/reduction catalyst without being trapped or stored by the catalyst during a lean-burn operation of the engine is increased.
The NOx stored in the NOx storage/reduction catalyst can be discharged from the catalyst for reduction and removal, through a rich-spike operation in which the engine is temporarily operated at a rich air/fuel ratio so that exhaust gas having a rich air/fuel ratio is supplied to the catalyst.
However, the above-mentioned SOx stored in the NOx storage/reduction catalyst is not discharged from the catalyst through the rich-spike operation for reduction and removal of NOx, and therefore the amount of SOx stored in the NOx storage/reduction catalyst gradually increases as the engine keeps operating at a lean air/fuel ratio, resulting in so-called sulfur poisoning or S poisoning, which causes a gradual reduction in the NOx removal capability of the NOx storage/reduction catalyst.
In addition to the S poisoning, the NOx storage/reduction catalyst may degrade due to, for example, a long period of use or operations under a high-temperature environment, resulting in a reduction in the NOx storage capacity.
The S poisoning may be eliminated by, for example, performing a poisoning eliminating operation in which a fuel-rich exhaust gas having a higher temperature than that used in normal rich-spike operations is supplied to the NOx storage/reduction catalyst so as to discharge the stored SOx from the NOx storage/reduction catalyst.
The NOx storage/reduction catalyst that has degraded for reasons other than S poisoning may not be recovered or restored to its original state, and the catalyst may need to be replaced by a new one in the case where such degradation occurs.
In order to appropriately eliminate the S-poisoning or replace the catalyst by a new one as described above, it is necessary to accurately determine that the NOx storage/reduction catalyst has degraded to such an extent that necessitates or requires the poisoning eliminating operation or replacement.
In a known technology for determining the degree of degradation of the NOx storage/reduction catalyst, a NOx sensor for detecting the NOx concentration in exhaust gas is disposed in an exhaust passage located downstream of the NOx storage/reduction catalyst, and the degree of degradation of the NOx storage/reduction catalyst is determined based on an output signal of the NOx sensor. Examples of exhaust emission control systems that determine the degradation of the catalyst in this manner are disclosed in, for example, Japanese Laid-open Patent Publications No. 7-208151 (JP-A-7-208151) and No. 2000-130212 (JP-A-2000-130212), and U.S. Pat. No. 6,167,695.
The system as disclosed in JP-A-7-208151 determines that the NOx storage/reduction catalyst has degraded when the time required for the exhaust NOx concentration detected by a NOx sensor located downstream of the NOx storage/reduction catalyst to increase to a predetermined level after a rich-spike operation is equal to or shorter than a predetermined time.
As the NOx storage amount increases, the NOx storage capacity of the NOx storage/reduction catalyst is reduced, and the amount of NOx that passes through the NOx storage/reduction catalyst without being trapped by the catalyst, out of NOx contained in the exhaust gas flowing into the catalyst, is increased. The NOx storage capacity thus reduced due to the increase in the NOx storage amount is normally restored through a rich-spike operation for reducing and removing the stored NOx. However, the SOx stored in the NOx storage/reduction catalyst is not discharged from the catalyst through a normal rich-spike operation, and therefore SOx remains in the catalyst even after rich spikes if the catalyst suffers from S poisoning, resulting in a reduction in the NOx storage capacity by a degree corresponding to the amount of SOx stored in the catalyst.
In the case where degradation of the catalyst occurs for other reasons than S poisoning, too, the NOx storage capacity is not completely restored even after rich spikes are executed. Namely, once the NOx storage/reduction catalyst undergoes degradation, the NOx storage capacity of the catalyst is not completely restored even after rich spikes. Upon occurrence of S poisoning, therefore, the NOx storage capacity of the catalyst is largely reduced due to absorption of even a relatively small amount of NOx after a rich-spike operation, and the amount of NOx that passes through the catalyst without being stored or trapped is increased within a short time.
The system disclosed in JP-A-7-208151 is adapted to measure the exhaust NOx concentration by means of the NOx sensor located downstream of the catalyst after a rich spike is executed, and determines that the NOx storage capacity is not sufficiently restored even after the rich spike, namely, the NOx storage/reduction catalyst undergoes degradation, when the time required for the NOx concentration to increase to the predetermined level is shorter than the predetermined time.
JP-A-2000-130212 discloses a technology for determining degradation of the NOx storage/reduction catalyst, by determining the amount of NOx emitted by the engine (which will be called “NOx emission amount”) based on the engine operating conditions, and comparing a value obtained by multiplying the NOx emission amount by the NOx absorption efficiency of the NOx storage/reduction catalyst that is determined depending upon the engine operating conditions, with the actual NOx concentration detected by a NOx sensor located downstream of the catalyst.
Namely, in the emission control system of JP-A-2000-130212, the amount of NOx (reference concentration) that would pass through the NOx storage/reduction catalyst and reach the downstream side thereof without being absorbed by the catalyst if the NOx storage/reduction catalyst is in a normal (i.e., non-degraded) state, out of the NOx generated by the engine, is determined as a product of the NOx emission amount of the engine and the NOx absorption efficiency of the catalyst. Then, it is determined that the catalyst undergoes degradation if the amount (concentration) of NOx that has actually passed through the catalyst to the downstream side thereof is larger than the reference concentration thus determined.
In the systems disclosed in JP-A-7-208151, JP-A-2000-130212 and U.S. Pat. No. 6,167,695, the degradation of the catalyst is determined based on the exhaust NOx concentration detected by the NOx sensor. However, these systems fail to take account of the reliability of the output of the NOx sensor (or the detection accuracy of the sensor), thus giving rise to a possibility that false or inaccurate determinations are made on the degradation of the catalyst.
In general, the NOx detection accuracy of the NOx sensor, namely, the reliability of the output of the NOx sensor, is considerably reduced or deteriorated in a low NOx concentration region. Nevertheless, when the degradation of the NOx storage/reduction catalyst is determined by the methods of JP-A-7-208151 and JP-A-2000-130212, a NOx-concentration judgment value based on which the degradation is detected needs to be set to a value in a relatively low concentration region.
Supposing that the upper limit concentration of the NOx emission level needs to be kept equal to or lower than, for example, 40 ppm, the purpose of determining degradation of the catalyst cannot be adequately accomplished if the occurrence of degradation is determined after the catalytic degradation has progressed to a point where the exhaust NOx concentration measured downstream of the catalyst has reached the upper limit concentration of 40 ppm. It is thus necessary to determine the progression of the degradation at an earlier point of time, and take measures, such as elimination of S poisoning, against the degradation.
For example, in the case where the degradation of the NOx storage/reduction catalyst is determined when rich-spike operations are periodically performed, if the exhaust NOx concentration measured downstream of the catalyst has reached 40 ppm at the time of a rich-spike operation, the emitted NOx concentration will far exceed the upper limit concentration under operating conditions (e.g., during acceleration of the vehicle) in which the NOx emission amount is far larger than that measured at the time of the rich-spike operation.
In order to constantly keep the exhaust NOx concentration equal to or lower than the upper limit value, therefore, it is necessary to determine that the degradation of the NOx storage/reduction catalyst has progressed (or the catalyst undergoes degradation) when the exhaust NOx concentration measured downstream of the catalyst immediately before a rich spike (i.e., upon a start of a rich spike) reaches a far lower value (e.g., about 10 ppm), and take measures, such as an operation to eliminate S poisoning, against the degradation of the catalyst.
Accordingly, in order to determine the degradation of the NOx storage/reduction catalyst at an earlier time in the methods of JP-A-7-208151 and JP-A-2000-130212, the degradation evaluation concentration as a criterion for determining degradation needs to be set to a sufficiently low level. However, the detection accuracy of the NOx sensor is considerably reduced in a low concentration region, as described above, and therefore the reliability of the degradation determination itself is reduced if the degradation evaluation concentration is lowered.
Thus, the conventional emission control systems are not able to accurately determine degradation of the NOx storage/reduction catalyst, and may cause a problem that the exhaust NOx concentration exceeds the upper limit value under operating conditions in which the NOx emission amount of the engine is increased.